A Calvary Coach bus carrying Pennsylvania high school students who had been touring Harvard University crashed Saturday night as it attempted to drive under an overpass in Boston and left dozens of passengers injured, four seriously, according to authorities.

The crash occurred after the bus hit the Western Avenue Bridge while traveling east on Soldiers Field Road around 7:34 p.m., authorities said. Firefighters, emergency medical services personnel, and State Police flocked to the scene to free passengers through the roof of the crumpled bus.

“They were doing a school tour,” he said. “They were heading back to Pennsylvania.”

MacDonald said the impact with the 10-foot high overpass, “pushed in the whole front of the roof.” He said four of the injured were removed through the roof and MBTA buses were used as a triage area to treat some of the injured passengers.

One victim had life-threatening injuries, and was taken to Boston Medical Center.

Three other victims were also seriously injured, according to Mehigan. One was transported to Brigham and Women’s Hospital, one to Beth Israel Deaconess Medical Center, and one to Massachusetts General Hospital, she said.

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The impact with the overpass pushed in the front of the roof of the bus, a Boston Fire Department spokesman said.

The 30 victims with minor injuries boarded an MBTA bus near the scene, Mehigan said. They were expected to be transported to Brigham and Women’s and Beth Israel hospitals. All victims were freed from the wreckage by 9 p.m., State Police said in a statement.

State Police spokesman David Procopio said it was not known whether the seriously injured victims were adults or students.

The driver will likely be cited for an overheight violation, said Procopio, and he could face more serious charges as well. The driver was not injured and remained at the site of the crash late Saturday night, where he was being interviewed by State Police.

The bus should not have been driving on the road because of its height, Procopio said. State Police believe the driver did not see the sign stating the bus was prohibited from traveling on the roadway, Procopio said.

Flashing blue and red lights colored the night sky at the accident scene, where more than a dozen firetrucks and ambulances were visible. Several emergency responders shepherded cars around the Charles River and Soldiers Field Road, but traffic remained heavily snarled nearby.

Soldiers Field Road was expected to remain closed until the bus was removed, said Procopio.

The gnarled wreckage of the bus remained on the side of the road late Saturday night. The roof was compressed and heavily bowed downward at the center. The windshield, badly cracked, remained in place, but the roof and sides around it were peeled back and missing.

Authorities at the scene said engineers will inspect the bridge, but there were no indications that it was structurally unsound.

Raymond Talmadge, owner of Calvary Coach, said late Saturday that he was working to arrange a bus to transport his customers back to Pennsylvania. Talmadge said he had spoken by phone with the driver of the bus, who was unharmed. He did not identify the driver, but said he had worked for Calvary for many years and was a “very, very good driver.”

Talmadge said in the 25 years he had owned Calvary, the company had never experienced a significant accident.

Data found on the website of the Federal Motor Carrier Safety Administration showed that Calvary is a small operation with two buses and two drivers. Calvary’s drivers had been inspected four times in the 24 months prior to Feb. 1, according to the website and there had been one vehicle inspection. The data showed no crashes in the past 24 months.

“It’s very traumatic,” said Hill, a member of the Destined for a Dream Foundation. “But we’re very grateful, it’s a situation that could have been a lot worse. It could have been a lot more horrifying that it was.”

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